In the course of drilling, or during production or workover, the vast majority of oil and gas wells are exposed to conditions that ultimately lead to formation damage. Formation damage limits the productive (or injective) capacity of the well. The reduction in well performance is generally due to changes in near-wellbore permeability which may be caused by a number of factors, such as rock crushing, invasion of drill solids, swelling of pore-lining clays, migration of mobile fines and changes in wettability.
It is known that permeability impairment may be improved by injecting acid formulations containing HF into the formation. Such methods are known to improve production from both subterranean calcareous and siliceous formations.
Most sandstone formation are composed of over 70% sand quartz, i.e. silica, bonded together by various amount of cementing material including carbonate, dolomite and silicates. Suitable silicates include clays and feldspars. A common method of treating sandstone formations involves introducing hydrofluoric acid into the wellbore and allowing the hydrofluoric acid to react with the surrounding formation. Hydrofluoric acid exhibits high reactivity towards siliceous minerals, such as clays and quartz fines. For instance, hydrofluoric acid reacts very quickly with authigenic clays, such as smectite, kaolinite, illite and chlorite, especially at temperatures above 150° F. As such, hydrofluoric acid is capable of attacking and dissolving siliceous minerals.
Upon contact of hydrofluoric acid with metallic ions present in the formation, such as sodium, potassium, calcium and magnesium, undesirable precipitation reactions occur. For example, during the treatment of calcareous or siliceous formations containing carbonate or dolomite, calcium or magnesium fluoride scales often form as a result of precipitation. Such scales tend to plug the pore spaces and reduce the porosity and permeability of the formation.
Alternative methods of treating calcareous or siliceous formations with hydrofluoric acid have been sought wherein the formation of undesirable scales is prevented or inhibited.